High rates of anxiety disorders (about 30-80%) reported in youth with autism spectrum disorders (ASD) reflect a growing public health concern. While these high rates have recently garnered attention in the literature, little is understood about the true nature of this phenomenon. The diagnostic interview schedules used to assess anxiety in youth with ASD have not been validated in the autism spectrum population, and it is unclear whether the high rates of comorbid anxiety disorders generated by these assessment instruments actually reflect clinical anxiety, or if they reflect a false positive result. It is possible that features of autism such as poor social relatedness and aloofness, or repetitive behaviors, are misclassified as anxiety by standard diagnostic interview schedules. If, on the other hand, the high rates of anxiety disorder diagnoses generated in these studies do reflect widespread clinically impairing anxiety in youth with ASD, this anxiety could result in greater functional impairment and a more severe course of the autism spectrum disorder. The goal of the proposed study is to use multiple methods of assessment, including structured behavioral observations and psychophysiological techniques, as well as clinician-, parent-, and youth-report, to test the validity of anxiety diagnoses generated by a widely used diagnostic interview schedule among youth with ASD. One hundred twenty children ages 7-14 years with a diagnosis of an ASD will be included. Eighty children will be recruited from the UCLA Center for Autism Research and Treatment (CART) and 40 children will participate as part of an overlapping cognitive-behavioral treatment (CBT) intervention for youth with ASD and comorbid anxiety. The specific aims of the project are to evaluate the concurrent validity of anxiety disorder diagnoses, and assess convergent and discriminant validity of clinical anxiety in youth with ASD by employing a multitrait, multimethod matrix approach as well as a cluster analytic framework. Additionally, the proposed work aims to test whether objective indices of anxiety improve corresponding with improvements in clinician, parent, and youth ratings of anxiety over the course of CBT for youth with ASD and comorbid anxiety. This work will contribute to the field by facilitating understanding of the nature and impact of anxiety in autism, thereby aiding in the development of more targeted and effective treatment strategies. These aims are consistent with the aspirational goals of the 2009 Interagency Autism Coordinating Committee Strategic Plan for Autism Spectrum Disorder Research, which stipulates the need for biobehavioral studies of ASD to identify and characterize co- occurring symptoms.